For the last two months we've been busy assembling the units, getting feedback and refining the details.
A major turnpoint was to scrap our diy head straps and use welding mask headgear instead. The problem was that you can't easily adjust the fit while wearing a mask, and the straps were not rigid enough to hold the shape while fitting.
The headgear both has a convenient adjustment knob, and is rigid enough. We've managed to source them locally for less than $2 a piece.
We've also iterated on the design of the adaptor for the hose, to make a reliable quick-release connection. We've tried 3d printing a bayonet mount, adapting various garden hardware and even considered injection molding, but none of that is cheap or simple to replicate.
A knobbed hose clamp is both cheap and secure so we'll call it good enough for now.
We've successfully finished a crowdfunding campaign to provide a total of 150 units to hospitals in Ukraine. We expect other makers and businesses to embark on the initiative once we receive eough feedback and release the source files and instructions (which we're struggling to finish now).
Keep safe everyone and don't hesitate to contact if you have any questions or feedback.
During the last month we've been receiving components and finalizing the technology.
We've decided to use two Ncr18650b batteries in parallel along with a controller that handles charging, steps up the voltage and protects the battery from overdischarge. A separate controller shows the remaining charge on an LCD display, and a power switch.
We've tested 4 different high-power blower fans and chose the smallest one. With our setup it gives 60 liters of air per minute (2.1 CFM) which is more than enough for breathing and accidental leakage.
The fan runs for 8 hours on a single charge.
As for the hood, we've added internal parts that limit the space around the face, providing more pressure. Initially we tried to sew the parts together, but welding polyethylene with a modified soldering iron worked much better.
This is the final look of the hood
With this simple rig, an oven and a shop vac we've iterated on a vacuum mold and eventually decided to make the first production run in-house.
We've assembled and tested the electronics for the first batch of 30 units
Having decided to omit CNC milling (as it's less popular and more expensive) and stick with vacuum forming and laser cutting, we've tried various plastics of different thicknesses. We've chosen 2mm white PETg as it is durable, forms easily, and can be bent after cutting.
The initial idea was to bend the marginal space around the mold inward, creating a strudy frame for the four bolts holding the filter.
We've considered adding rivet nuts, captive nuts or pieces of aluminium tube but tried to keep it simple.
After several iterations we've settled with two-piece design, with simple bolts holding it all together.
Videos of the process coming soon.
In the meantime, we're building a test chamber and finalizing the hood design to hopefully release the project in a week.
The box contains a HEPA filter, a 12V fan, and 3 cell 18650 Li-ion batteries along with a BMS circuit. We've tested the setup work for over 10 hours which is more than enough for a working day.
Here's a mock-up of the electronics. At first we used cheap charge indicator modules but that wouldn't be as readily available as the rest, so we set out for a custom PCB.
We've designed a circuit with a speed controller and charge indicator with Arduino Nano as a controller, but found the soldering part too unreliable and time-consuming. We are now looking for an alternative solution.
This is the design so far. The filter fits inside and is held tightly with a plastic frame. Holes are sealed with hot glue.
A small number of units was produced and given out to three local hospitals to test.
We worked through several iterations, starting with a rough 3d model. We went for one-size-fits-all approach.
Layers of plastic film tend to weld together when laser cutting. Developing that idea, we ran an offset contour with defocused beam to create a reliable seal, and focused the laser to cut the final shape. Later we found that technique used in a hackaday article.
It was extremely difficult to find nice looking non-transparent PE film, but we've found inexpensive polyethylene bags. Thickness between 100 and 200 microns works best for us.
The visor is attached with scotch tape. We found that natural rubber adhesive tape is most resistant to alcohol and can be sanitized multiple times.
The adjustable head band and visor are made of PETg plastic. The hose is a standard sanitary item, and we've found a similar size across multiple manufacturers.
We've decided to use an HEPA certified filter to provide a high level of protection. A vacuum filter was the cheapest solution, as custom-made units would cost at least twice as much.
We ran a series of tests with different fans and studied professional PAPR units. To get a good enough airflow we needed a static pressure of about an inch (25mm) in H20. So we've chosen a fitting blower fan and added three 18650 cells to get about 12V. At this point we used a charge indicator module and were searching for an affordable battery manager/charger.
The initial idea was to build a low cost protection without wearing something on your face. Helthcare workers during the COVID-19 pandemic work long shifts, and anything attached to your mouth is annoying at least.
We've built the first prototype by surrounding a face shield with polyethylene film and attaching a PC fan on top. This has worked relatively well, but the shield is heavy and there is no real filtering. There is definitely room for improvement.